The invention relates to a feed additive, and particularly relates to a microbial fermentation additive and a preparation method as well as application thereof. The preparation method of the microbial fermentation additive comprises the following steps of mixing bacillus licheniformis, bacillus subtilis, bifidobacterium bifidum, enterococcus faecalis, lactobacillus acidophilus, lactobacillus casei, lactobacillus lactis, lactobacillus plantarum, pediococcus acidilactici, pediococcus pentosaceus, candida utilis, saccharomyces cerevisiae and rhodopseudomonas palustris microbial inoculum according to a proportion by a viable count per unit mass, which is (500 to 600): (500 to 600): (10,000 to 75,000): (10,000 to 73,000): (10,000 to 75,000): (10,000 to 73,000): (10,000 to 73,000): (10,000 to 65,000): (10,000 to 60,000): (10,000 to 75,000): (10,000 to 75,000): (50 to 70): 1, and obtaining the microbial fermentation additive through preparation. According to the microbial fermentation additive and the preparation method as well as the application thereof, straw is subjected to fermentation treatment through a synergistic effect among strains; the fermentation level of the straw can be quickly improved; the deficiency that a single strain secretes enzymes and synthetics is made up; the fermentation quality is improved; meanwhile, by generating many kinds of organic acid and mycoprotein, the palatability of the straw can be improved; the feed intakes of livestock and poultry are improved; the production properties of the livestock and the poultry are obviously improved.